1. Field of the Invention
The present invention relates to a method of manufacturing flash memory devices, and more specifically, to a method of manufacturing flash memory devices, wherein the device mass production capability, erase/write (E/W) cycling characteristics and bake retention characteristics can be improved.
2. Discussion of Related Art
As the cell size shrinks and the operating voltage is lowered due to higher integration, scale-down of a tunnel oxide film is required in order to secure cell characteristics of flash memory devices.
There is, however, a limit to the scale-down of the tunnel oxide film due to data retention reliability problems. In order to overcome this limitation, an oxynitride film (N2O) having better properties than a silicon oxide film (SiO2) in the same thickness has been used as the tunnel oxide film instead of the silicon oxide film.
In the case where the tunnel oxide film is fabricated according to conventional technology, however, it is difficult to control a thickness and the nitrogen concentration of the tunnel oxide film, which have a great influence on the reliability, in some regions within a batch where the tunnel oxide film process proceeds. This causes the thickness of the tunnel oxide film and the nitrogen concentration of the tunnel oxide film to be profound depending upon locations within the batch. As a result, since a cell threshold voltage and characteristics vary, satisfactory erase/write (hereinafter, referred to as “E/W”) cycling and bake retention characteristics cannot be obtained.
Meanwhile, in order to form a tunnel oxide film having a uniform thickness and nitrogen concentration, a tunnel oxide film process has to be performed only within a specific region of the batch. The number of lots, which can be actually processed in a batch in which 5 lots can be processed by maximum, is at most 2. Accordingly, the mass production capability is low.
Conventional problems will be below described in more detail with reference to FIGS. 1 to 3.
FIG. 1 is a table showing a thickness and a nitrogen concentration of a tunnel oxide film on a batch basis, which is fabricated according to the prior art.
Referring to FIG. 1, a tunnel oxide film process is performed in a region U within the batch, but is not carried out in regions C and L. This is because it is difficult to control a thickness and a nitrogen concentration of the tunnel oxide film in the regions C and L only through the conventional method of manufacturing the tunnel oxide film. Accordingly, although a maximum number of lots that can be processed within a single batch is 5, the number of lots that can be processed actually is at most 2 since the process is carried out only in the region U except for the regions C and L. Thus, the mass production capability is low.
As well known in the art, in the flash memory device, data are erased or written (or programmed) by injecting or drawing electrons into or from the floating gate by way of F-N tunneling. Meanwhile, when data are read, the status of a cell, i.e., a program or erase status is decided according to whether electrons exist in the floating gate or not.
As such, in the repetitive F-N process where data are programmed and erased, a cell threshold voltage is changed since electrons are trapped in the tunnel oxide film. Upon read, there arises a problem in that data stored in a cell are erroneously recognized. It is thus required that error in recognizing data not occur during at least 10K E/W (erase/writing) cycling.
FIG. 2 is a graph showing 10K E/W cycling characteristics of a conventional flash memory device.
From FIG. 2, it can be seen that after 10K E/W cycling, a program threshold voltage P shifts about 1.0V from 0.2V to 1.2V, and an erase threshold voltage E shifts about 2V from −3.8V to −1.8V. That is, excessive shift in a threshold voltage occurs. Such excessive shift in the threshold voltage is caused since the amount of charges trapped in the tunnel oxide film fabricated according to the present invention increases during 10K E/W cycling. It is thus required to improve the film quality of the tunnel oxide film.
FIG. 3 shows the test results of bake retention characteristics after 10K E/W cycling of the conventional flash memory device.
A program threshold voltage of a flash memory device in an initial state where the device is not used and a program threshold voltage of the flash memory device after 10K E/W cycling is 1.0 to 3.0V. In contrast, if a bake process is performed, for example, for 24 or 48 hours after 10K E/W cycling, as shown in FIG. 3, the range of the threshold voltage shifts about 1.0V. The program threshold voltage becomes 0.0 to 1.5V. Considering that the range of the threshold voltage upon program is 1.0 to 3.0V in a typical NAND flash memory device, the threshold voltage margin is at most 0.5V after the bake process. Thus, fail occurs in the device operating characteristics.
The main cause of the fail is that the threshold voltage is lowered since electrons trapped in the tunnel oxide film during 10K E/W cycling are de-trapped after the bake process.